One idea could be that Harry discovers an ancient spell book in the Room of Requirement that has the incantations for creating a magic railgun. He might use it to defend Hogwarts against a new, powerful dark force that is immune to traditional magic spells.
The new wand might have a core that is a combination of multiple magical elements. For example, a strand of a unicorn's hair combined with a shard of a powerful magical crystal. This would make the wand extremely versatile, able to channel different types of magic depending on how the wizard holds it and the incantation used.
He could collaborate with Ollivander. Ollivander is the expert in wands, and he has a vast knowledge of wand - making materials and techniques. Working together, they could combine their knowledge to create something truly extraordinary.
Well, if Harry Potter creates his own magic in fanfiction, it could lead to him being either celebrated or feared. On one hand, his new magic could solve many of the problems the wizarding world faces. But on the other hand, the Ministry of Magic might see it as a threat to their control. They could try to regulate or even suppress his new magical creations.
One idea could be that Harry creates a school for magical creatures. He realizes that there are so many unique and often misunderstood creatures in the wizarding world, and he wants to give them a place to learn and grow. This school could teach them how to interact with wizards in a positive way, as well as develop their own magical abilities.
Harry may decide to have a more intuitive magic system. Instead of relying on wands, which can be lost or broken, the magical beings on his planet could use their minds and emotions to cast spells. The stronger the emotion or intention, the more powerful the spell. And to prevent misuse, there could be a natural balance in the planet's magic. If someone tries to use dark magic, the planet itself could resist and weaken the spell, teaching the user that dark magic is not welcome.
The exploration of the wizarding world is a major part. When Harry creates magic, it can open up new areas of the wizarding world to be explored. It could be a hidden chamber where the new magic was born or a new group of wizards who are interested in his creations. This allows the author to expand on the already rich Harry Potter universe and take the readers on a new and exciting journey.
In this fanfiction, Harry might also include some of the Hogwarts ghosts in his army. After all, they have a long - standing relationship with the school. Peeves, for example, could be a wild card in the army. He can cause chaos among the enemies. And some of the older students who have a strong sense of justice, like Cedric Diggory's friends, could also be part of it.
I'm not sure if it's a new plot trend. Fan culture is so diverse that there could be some individual works exploring this idea, but it might not be widespread enough to be considered a trend. It could be a unique take by a particular author or group of fans who want to explore a different aspect of the Harry Potter universe.
The plot could start with Harry finding an ancient Jedi artifact during one of his adventures. This leads him to become curious about lightsabers. He then uses his magic knowledge to try and replicate it. Along the way, he might face opposition from some in the wizarding world who think it's too dangerous or against wizarding traditions. But Harry, being the hero he is, persists. Eventually, he creates a lightsaber and uses it to defeat a new, powerful dark wizard who has emerged, using both his wizarding skills and his new lightsaber skills.